The present invention relates generally to the design and construction of an integrated system for controlling rotor positioning within a compressor for improved performance and operability of a gas turbine engine. More particularly, the present invention has one application wherein radial magnetic bearings are utilized to control rotor displacement within a compressor to delay the onset of rotating stall. Although, the invention was developed for use in a gas turbine engine, certain applications may be outside of this field.
It is well known that a gas turbine engine must have a compressor component that develops some or all of the pressure rise specified by the system cycle. Modern designers of gas turbine engines have generally utilized axial flow compressors having an internal flow through an annular flow path, where the flow is influenced by both rotor and stator blade rows. The blade rows operate in an unsteady flow, where both the velocity magnitude and direction fluctuate. Further, blade rows are generally designed so as to behave as diffusers, with an increase in static pressure from the upstream to the downstream region. The individual blades are subject to lift and drag forces, they stall, and they generate boundary layers, wakes and under some circumstances shock waves.
Rotating stall and surge are two fundamental compressor instability phenomenon that set an absolute limit to the compressor operational range. With reference to FIG. 1, there is illustrated an equilibrium performance map for a compressor. At each rotational speed, the mass flow rate was varied between two limits. The upper limit on mass flow rate being close to the maximum obtainable for the compressor at that speed. The lower limit being fixed by one of these aerodynamic instabilities, surge or rotating stall. Surge is manifested by large-scale flow instability, often with pulsating reversal of flow involving the entire unit. In a gas turbine engine, compressor surge may produce catastrophically high levels of vibration and loud acoustic emissions. A surging engine operates in a low frequency limit cycle varying from the non-surging power level to a greatly reduced power level. Individual instability points at the various rotational speeds define a surge line that is universally considered as a boundary for acceptable compressor operation.
Surge occurs simultaneously throughout the entire extent of the compressor flow field, both circumferentially and axially. However, one or more blade rows may be locally stalled without the occurrence of surge. Stall is manifested by significant regions of separated flow which cause major changes in the pressure distribution around the individual compressor blade. Stall in a blade row is unique in a number of aspects, especially in the fact that zones of flow separation can propagate from blade to blade in the annulus, resulting in a phenomenon known as rotating stall. Rotating stall can also lead to high vibration levels in the compressor which ultimately result in mechanical failure. An engine in rotating stall operates at a steady, but greatly reduced level from the non-stalled power level. Compressor designers in order to obtain optimum performance often desire to operate the apparatus on the verge of stall; therefore it is important to be able to control compressor stabilities in order to delay the onset of rotating stall.
A number of passive techniques, i.e., tip casing treatment have been used to extend the operating range (displace the surge line to a lower flow level at a constant rotational speed) with varying degrees of success. Active feedback techniques have been utilized to alter the dynamics of the aerodynamic system and displace the surge line. The conventional active feedback techniques include air injection systems and oscillating guide vanes. With reference to FIG. 2, there is illustrated a conventional air injection system. Working fluid from the compressor is injected into the flow through a series of nozzles and valves located around the circumference of the compressor. The pressurized fluid is best characterized as an injected disturbance that is utilized to dampen the localized flow separations occurring around the circumference of the annular flow path, and delay or prevent the onset of rotating stall.
The air injection systems and oscillating guide vanes involve adding actuators (mechanical, aerodynamic, and aeromechanical) controllers, and sensors to the gas turbine engine. Although in many instances these technologies have demonstrated the potential for substantial benefit, they also have significant limitations. One limitation being that the required hardware adds weight and can be difficult to integrate into existing engine designs. A second limitation being that inherently these systems deliver the injected disturbance at a finite number of locations, thereby not directly altering the dynamic behavior of the fluid across the entire flow region.
Although the prior techniques utilizing air injection systems, or oscillating guide vanes are steps in the right direction for enhanced compressor stability control, the need for additional improvement still remains. The present invention satisfies this need in a novel and unobvious way.